Method of measurement accuracy improvement by control of pattern shrinkage

ABSTRACT

A scanning method for a scanning electron microscope is provided which minimizes a degradation in dimension measuring accuracy caused by a shrink of a specimen. A time between the first and the second scan over the same location on the specimen is shortened by changing the scanning order of scan lines to enable the scanning to be performed successively while the shrink is small.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning electron microscope toobserve a fine pattern to measure its dimensions and more particularly ascanning electron microscope designed to take measurements of sampleswhose shape changes as they are applied with an electron beam.

In a manufacturing and inspection process of functional elementproducts, such as semiconductor devices and thin-film magnetic heads,which are fabricated by a surface microfabrication technique, a scanningelectron microscope is widely used for measurement of widths of patternsfabricated (referred to as “critical dimension measurements”) and forexternal inspection. The scanning electron microscope forms an image ofa sample in a sequence of steps that involves: linearly ortwo-dimensionally scanning an electron beam, which is emitted from anelectron source and finely focused by a converging lens or objectivelens using interactions between a magnetic field or electric field andthe electron beam, over the sample using a beam deflector; detecting asecondary signal (secondary electrons and back-scattered electrons)produced by the electron beam by using a photoelectric effect-baseddetector; and transforming the detected signal into a visible signal,such as a luminance signal synchronous with the scanning of the electronbeam.

In scanning electron microscopes, provisions are made to ensure that animage of a specimen obtained depicts, with high precision, features of aspecimen surface being observed and measured. That is, when the specimensurface is examined, visualization points of the image signal arearranged at positions precisely similar to those positions of thecorresponding points in an area being scanned. To realize this positionmatching, the scan area and the image area are both set rectangular inshape and made up of the same number of scan lines whose length is equalto one side of the rectangle. Generally, the scan area and the imagearea are set to have the same ratio of a scan line length to a distancebetween the adjoining scan lines. With this arrangement, a distancebetween any two points on the specimen surface has a constant ratio to adistance between the corresponding two points in the image of thespecimen at all times. This ratio signifies a magnification of thescanning electron microscope. Such a technology has been widelyimplemented as a basic technology in the scanning electron microscopes,as described in L. Reimer, “Scanning Electron Microscopy”,Springer-Verlag Berlin and Heidelberg GmbH & Co. KG, 1985, page 2, forexample. From the specimen image thus obtained, the distance between anytwo points on the specimen surface can be calculated easily. Thiscalculation is generally called “critical dimension measurement” and ascanning electron microscope with such a calculation function is calleda “critical dimension scanning electron microscope.”

An example case where the scan area on the surface of a specimen and thecorresponding specimen image are not similar is described inJP-A-2001-147112. In this example, for a specimen with patterns sospaced apart as to make it necessary to measure dimensions with areduced magnification although the patterns are very fine, a secondaryelectron image is formed by extending the specimen image in a directionperpendicular to a line connecting two points on the specimen, therebyimproving the measurement accuracy.

Such a scanning electron microscope therefore needs to radiate againstthe surface of the specimen being probed an electron beam with a surfacearrival energy of several hundred electron bolts.

As features of microfabricated semiconductor surfaces have become moreand more miniaturized in recent years, a photoresist that reacts to anargon fluoride (ArF) excimer laser beam (referred to as an “ArF resist”)has come to be used as a photosensitive material for photolithography.The ArF laser beam has a short wavelength of 193 nm and thus the ArFresist is considered suited for exposing finer circuit patterns. Ourrecent study, however, has found that the ArF resist is very fragilewhen subjected to electron beams and that, when a fabricated pattern isprobed or its critical dimensions are measured by a scanning electronmicroscope, a base material such as acrylic resin is condensed by thefocused electron beam, reducing its volume (referred to as a “shrink”)and deforming the circuit pattern.

To realize a design performance of semiconductor devices requires astringent control of shapes and dimensions of circuit patterns and, forthis reason, a critical dimension scanning electron microscope capableof measuring very fine dimensions is used in an inspection process.However, if the shape of a pattern changes upon application of acritical dimension measuring electron beam during the observation andmeasurement processes, an expected design value of the circuit patterndimension cannot be realized, leading to a problem of degradation ofdevice characteristics and failures. Further, since line widths changewhen subjected to an electron beam, a measured value of the samedimension may vary each time a measurement is made, making it impossibleto improve the measurement accuracy. At present, no other equipment thanthe critical dimension scanning electron microscope is available thatcan measure fine dimensions at a satisfactory precision. So, the patternshrink poses a serious problem to the semiconductor device manufactureusing an ArF resist. Conventional scanning electron microscopes, asdescribed above, do not provide a means to deal with the shrink of aspecimen caused by the electron beam and there is a problem withmeasured values of pattern dimensions. Further, in the case of theJP-A-2001-147112 described above, although the measurement accuracy of adimension between two separate points on a specimen is taken intoconsideration, no provisions are made to deal with the shrink ofspecimen caused by the application of an electron beam.

It is an object of this invention to provide a scanning electronmicroscope capable of measuring a pattern formed on a specimen thatshrinks upon being radiated with an electron beam, such as an ArFresist.

The shrink of an ArF resist pattern is considered to be a chemicalreaction caused by the energy of a focused electron beam incident on theresist. It is thus considered that the volume of the ArF resist willchange after the irradiation of electron beam according to an equation(1) below.V _(s) =V exp(−t/τ)  (1)where Vs is a volume of the ArF resist after being irradiated with anelectron beam, V is a volume of the ArF resist before the application ofthe electron beam, τ is a time constant of the chemical reaction in theArF resist, and t is an elapsed time.

As can be seen from equation (1), there is a time lag from when anelectron beam is radiated against the ArF resist until the resistshrinks by a reaction. When a scanning electron microscope is used tomake measurements, since an image formed in only one scan (1 frame) hasa poor S/N ratio, it is common practice to overlap a plurality of framesand produce an averaged image to improve the S/N ratio and therebyenhance the measurement accuracy. Based on the fact that (1) there is atime lag from an instant the ArF resist is irradiated with an electronbeam until a reaction takes place and that (2) normally a plurality offrames are overlapped to form a target image for dimension measurement,we have studied measures to deal with the problems described above.

Suppose a time which elapses from when an electron beam scan isperformed once over a specimen until the next scanning electron beamreaches the same position on the specimen is Ti and that a time constantof the chemical reaction of the ArF resist is τ according to equation(1). In an interlaced scan at 60 Hz, Ti was 33 ms and, in an experimentperformed by the inventor, τ was 5 ms. At this time, Ti>τ and the secondelectron beam scan is executed after the shrink of the ArF resist hasprogressed significantly (see FIG. 1A). On the contrary, this inventionhas Ti<<τ (see FIG. 1B). By shortening the scan interval, it is possibleto make precise measurements with a reduced amount of shrink.

That is, a highly accurate measurement is made of a resist such as anArF resist that shrinks when subjected to an electron beam, by changinga scanning order of scan lines to shorten the time between the first andthe second scan over the same location on the specimen to allow thescans to be performed successively while the amount of shrink is small.The time between the scans can also be shortened by reducing the numberof scan lines or shortening the scan width. Further, a user is providedwith an environment that makes for an easier use of the microscope byregistering parameters, such as scanning order, the number of scanlines, scan width and the total number of frames, as fixed values inadvance so that the user can choose a desired combination of these attime of taking measurements.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a scanningelectron microscope comprising: an electron beam source; an electronbeam scanning unit to focus an electron beam emitted from the electronbeam source and sweep it over a specimen; a detector to detect secondarysignals coming out from the specimen when it is subjected to theelectron beam; and an imaging unit to form a specimen image by summingup a plurality of image signals based on the secondary signals detectedby the detector; wherein a plurality of electron beam scanning patternsare selectably provided in which the electron beam is scanned over thespecimen by the electron beam scanning unit; wherein the plurality ofscanning patterns have different time periods which elapse after eachelectron beam radiation position in a scan area on the specimen hasreceived the electron beam until it receives the beam again. A typicalscanning pattern is to repeat as many scans over one scan line as thetotal frame number before moving to the next scan line. The imaging unitmay form a specimen image by summing up a plurality of frames of imagesignals.

This invention permits highly precise dimension measurements forspecimens, such as photoresists for an argon fluoride (ArF) excimerlaser beam, that are deformed by an electron beam. Further, since thisinvention does not change the scanning speed of the beam, no significantmodification in the equipment structure is needed and therefore thisinvention can be implemented by only software modification and easilyintroduced to conventional equipment.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A-1B are diagrams showing a relation between scanning methods andthe amount of shrink.

FIG. 2 is a schematic diagram showing one example configuration of ascanning electron microscope according to this invention.

FIG. 3 shows an example of a scanning method selection page.

FIGS. 4A-4C show how a normal scanning (interlaced scanning) isperformed.

FIGS. 5A-5C show how a continuous scanning is performed.

FIGS. 6A-6C show how a scanning is performed when the number of scanlines is changed.

FIGS. 7A-7C show how a scanning is performed when a scan width ischanged.

FIGS. 8A-8C show how a scanning is performed when both the scan linenumber and the scan width are changed.

FIGS. 9A and 9B show an example case of measuring a hole pattern.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 is a schematic view showing an example construction of a scanningelectron microscope according to this invention. An acceleration voltagefrom a high voltage control power supply 104 controlled by a controlprocessor 119 is applied between a cathode 101 and a first anode 102 todraw a predetermined emission current from the cathode 101. Between thecathode 101 and a second anode 103 is impressed the acceleration voltagefrom the high voltage control power supply 104 controlled by the controlprocessor 119, so a primary electron beam 110 emitted from the cathode101 is focused by a converging lens 105 controlled by a converging lenscontrol power supply 106 and then removed of an unwanted peripheralportion by a throttle plate 107.

Then, the primary electron beam 110 is focused into a fine spot by anobjective lens 111 controlled by a objective lens control power supply112. The beam spot is swept two-dimensionally over a specimen 113 by adeflection coil 108. A scan signal for the deflection coil 108 iscontrolled by a deflection coil control power supply 109 according to amagnification. The specimen 113 is securely placed on a specimen stage114 that can be moved two-dimensionally. The movement of the specimenstage 114 is controlled by a stage controller 115.

Secondary electrons 116 emitted from the specimen 113 when struck by theprimary electron beam 110 are detected by a secondary electron detector117 and amplified by a secondary electron signal amplifier. An imagingunit 120 transforms the detected secondary signals into visible signalsthat are arranged on a separate plane and shows on a display unit 121 animage that precisely represents a surface topography of the specimen. Aninput unit 122 provides an interface between an operator and the controlprocessor 119. Using the input unit 122, the operator controls variousunits, specifies points to be measured and issues a command fordimension measurement.

While in FIG. 2 the control processor has been described to beintegrated in the scanning electron microscope or configured in asimilar manner, other configurations may also be employed. For example,the control processor may be arranged as a separate unit from thescanning electron microscope and assigned with processing described inthe following. In that case, additional components are required, whichinclude: a transfer medium that transfers signals detected by thesecondary electron detector 117 to the control processor or transferssignals from the control processor to the lenses and deflection coil inthe scanning electron microscope; and input/output terminals that inputand output signals via the transfer medium. It is also possible toregister with a memory unit 123 a program for performing the processingdescribed below and have the program executed by the control processor119 that has an image memory and supplies necessary signals to thescanning electron microscope.

Measurement of a pattern dimension involves, for example, having twovertical or horizontal cursor lines displayed on the display unit 121together with a specimen image and aligning the two cursors with patternedges by operating the input unit 122. Based on information about adistance between the two cursors, the control processor 119 calculates ameasured value as a dimension of the pattern.

Stored in the memory unit 123 is information on a scanning pattern whichincludes a plurality of control data for determining the order of scan,a plurality of control data for determining the number of scan lines, aplurality of control data for determining a scan width, and a pluralityof values representing the total numbers of frames. When starting themeasurement, an operator selects and specifies, from among scan methodsdisplayed on a scan method selection page, a desired condition suitedfor a specimen to be measured including the scan order, scan linenumber, scan width and total frame number. With a desired scan methoddetermined, the control processor 119 retrieves control datacorresponding to the selected condition from the memory unit 123 and, byusing the control data retrieved, controls the deflection coil controlpower supply 109 to sweep the electron beam in a desired scanningpattern over the specimen. Initiating the measurement as described aboveallows an image to be retrieved according to the scanning methodmodified by the user and therefore a precise measurement to be madebased on the retrieved image.

FIG. 3 shows an example scanning method selection page. On this scanningmethod selection page the user can selectively specify a desiredelectron beam scanning method used in creating an image of the sample.Shown on this scanning method selection page as selection items arescanning order, scan line number, scan width and total frame number.FIG. 3 shows that a continuous scanning is chosen as the scan order,normal 512 lines as the number of scan lines, {fraction (1/1)} as thescan width, and 8 as the total number of frames.

First, the “scan order” will be explained. “Normal (interlacedscanning)” shown as one choice of scanning order is an interlacedscanning normally used in conventional scanning electron microscopes,and “continuous scanning” is the one used in this invention. Adifference in the scanning order between the conventional normalscanning (interlaced scanning) and the continuous scanning of thisinvention will be explained by referring to FIG. 4 and FIG. 5.

FIG. 4 explains the order of scanning during the normal scanning(interlaced scanning). As shown in a monitor screen diagram of FIG. 4A,the normal scanning is the same interlaced scanning as used in TV. Letus take an example case of 512 scan lines. In this example, a first scanis made in the horizontal direction from upper left corner of the screento upper right corner. The similar scanning operation is repeated bymoving the starting point of scan vertically down until 256th scan isexecuted. Then, the scan starting point is moved up to a positionbetween the first scan line and the second scan line where a 257th scanis executed. This is followed by a 258th scan performed between thesecond scan line and the third scan line. Such a scan is repeated until512th scan is completed, thus forming one frame of image. While theorder of scan has been explained on the monitor screen, the specimen isalso scanned by an electron beam in the same order as described above.With this scanning method, in the case of 60-Hz TV scan one frame takes33 ms. For example, when eight frames of image signal are combined toform an image, this interfaced scanning is repeated eight times.

FIG. 4B and FIG. 4C schematically show a vertical synchronization signaland a horizontal synchronization signal for the normal scanning(interlaced scanning). Both the vertical synchronization signal and thehorizontal synchronization signal are basically of sawtooth waveform.While the vertical synchronization signal describes one sawtooth wave,the horizontal synchronization signal describes 256 sawtooth waves. Thisis repeated two times to form one frame of image.

However, in the case of a specimen which may sustain physical orchemical changes when subjected to an electron beam during a pluralityof scans of the above scanning method, such as an ArF resist thatshrinks upon beam application, pattern dimensions may change due toirradiation of the electron beam, making a precise, stable measurementof dimensions impossible. A conventional practice to reduce a shrink ofresist due to application of an electron beam requires reducing theamount of electron beam emitted or lowering the measuring magnification,which in turn gives rise to other problems of a degraded S/N ratio ofsecondary electron signal and lowered measuring and reproductionprecisions caused by a reduced magnification. One of scanning methodsthat can avoid these problems is the continuous scanning adopted by thisinvention.

Referring to FIG. 5, the continuous scanning of this invention will bedescribed. FIG. 5A shows the order of scan lines on the monitor screenfor the continuous scanning. FIG. 5B and FIG. 5C show a verticalsynchronization signal and a horizontal synchronization signal, bothused to realize the continuous scanning. Here, an example case is takenup where one frame is made up of 512 scan lines and eight such frames ofimage signal are combined to produce a measured image.

In the continuous scanning, the deflection coil control power supply 109controls a scan signal for the deflection coil 108 to sweep the beamalong as many scan lines as used in the conventional method, 512 scanlines per frame, in an order shown in FIG. 5A, which differs from theconventional order of scanning and in which the scanning is performedalong each scan line as many times continuously as the number of framesto be combined. That is, on the monitor screen, the uppermost scan lineis first scanned for eight frames continuously. Next, the scanning startpoint is moved to the second scan line which is then scanned for eightframes continuously. In this way each scan line is scanned as many timessuccessively as the number of frames to be combined. In the final step,the bottom scan line is scanned for eight frames continuously. The imageof the sample is formed by combining the eight frames of pixel signalsfor each scan line. While the order of scanning has been explained byreferring to the monitor screen, the electron beam is also scanned overthe sample in the order described above. When the total number of framesper image is changed, the number of times that each of the scan lines isscanned successively also changes. When, for example, the total framenumber is changed to four, each of the 512 scan lines is scanned fourtimes in succession before moving to the next scan line, and theresulting image signals are used to form an image of the specimen.

Compared with the conventional method, this continuous scanning has thesame magnification but the time it takes for the beam to reach the samelocation on the specimen a second time after the first time is reducedto one 512th. If a 60-Hz scanning is considered, the time Ti whichelapses from the instant a certain point on the specimen is scanned (byan electron beam) until it is scanned again is 33 ms in the conventionalnormal interlaced scanning method, whereas the continuous scanningreduces the time Ti to about 0.064 ms, which is sufficiently smallerthan the chemical reaction time constant for ArF resist, τ=5 ms, torealize a target condition of Ti<τ. Therefore, in the scanning processthat overlaps a plurality of frames to produce an image of the specimen,since the scanning to retrieve image signals is done while the shrink issmall, the measurement accuracy can be made higher than whenmeasurements are made by using a specimen image formed by theconventional interlaced scanning of an electron beam.

The horizontal synchronization signal for the continuous scanning issimilar to the one used in the conventional interlaced scanning, asshown in FIG. 5C. However, the vertical synchronization signal differsfrom that of the interlaced scanning and has a steplike waveform, asshown in FIG. 5B. In one step of the vertical synchronization signal, asmany horizontal synchronization signals as the total frame number (e.g.,eight horizontal synchronization signals when the total frame number iseight) are produced.

Returning to the scanning method selection page of FIG. 3, a procedurefor setting the number of scan lines will be explained. In the exampleof FIG. 3, the number of scan lines making up one page of image can bechosen from among normal (512 lines), ½ (216 lines), ¼ (128 lines) and ⅛(64 lines). By controlling the scan signal for the deflection coil 108by the deflection coil control power supply 109, the number of scanlines making up one page of image can be reduced to shorten the time ittakes to retrieve one frame of image signals. That is, the time intervalcan be shortened which elapses from the moment an image signal isretrieved for a certain frame until an electron beam is radiated againstthe same location on the specimen to retrieve an image signal for thenext frame. This method for reducing the number of scan lines iseffective when measuring line widths of, for example, a line pattern.

FIG. 6 shows an order in which the scan lines are scanned when 64 lines,one eighth of the normal number, are chosen as the number of scan lines.Here, our explanation assumes that the normal (interlaced scanning) isselected as the order of scanning and {fraction (1/1)} as the scanwidth. FIG. 6A shows a range of image displayed on the monitor screenand an order of scanning. The monitor screen is so set that the specimenimage is displayed on the entire screen when the number of scan lines is512. If the number of scan lines is set to 64, one eighth of 512, avertical dimension of the displayed image is one eighth of that of themonitor screen. Although the scanning order and the scanning range havebeen explained on the monitor screen, the electron beam is also sweptover the specimen in a scan range that is narrowed in the sub-scandirection. Changing the number of scan lines naturally changes the widthof the scan area in the sub-scan direction. For example, when the numberof scan lines is set to ½, the vertical dimension of the image displayedon the monitor screen is ½ of the monitor screen size.

FIG. 6B and FIG. 6C schematically show a vertical synchronization signaland a horizontal synchronization signal, respectively, during thescanning operation. Since the number of scan lines is set to 64 lines,one eighth of the normal 512 lines, the amplitude of the verticalsynchronization is one eighth of that when the normal 512 lines areused. The amplitude of the horizontal synchronization signal is the sameas that used in the conventional scanning method. By setting theamplitude of the vertical synchronization signal smaller than normal,the period of the vertical synchronization signal is shortened, which inturn reduces the time required to retrieve one frame.

Returning again to the scanning method selection page of FIG. 3, aselection of the scan width will be explained. In the example of FIG. 3,the scan width can be chosen from among {fraction (1/1)} (normal), ½ and¼. The scan signal for the deflection coil 108 can be controlled by thedeflection coil control power supply 109 to change the scan width toreduce a period of the horizontal synchronization signal. This in turnshortens the time required to retrieve one frame of image signals andtherefore the time interval which elapses from the moment an imagesignal is retrieved for a certain frame until an electron beam isradiated against the same location on the specimen to retrieve an imagesignal for the next frame.

FIG. 7 schematically shows how the scan lines are scanned when ½ thenormal scan width is selected. As for other selection items, the normal(interlaced scanning) is chosen as the order of scanning and the normal512 scan lines are chosen as the number of scan lines. FIG. 7A shows arange of image displayed on the monitor screen and an order of scanning.The monitor screen is so set that the specimen image is displayed on theentire screen when the scan width is {fraction (1/1)} (normal). When thescan width is set to ½, the horizontal width of the image displayed isone-half the width of the monitor screen. While the scanning order andthe scanning range have been explained on the monitor screen, theelectron beam is also swept over the specimen in a scan range that isreduced in the main scan direction.

FIG. 7B and FIG. 7C schematically show a vertical synchronization signaland a horizontal synchronization signal, respectively, during thescanning operation. Because the scan width is set to ½ the normal scanwidth, an amplitude of the horizontal synchronization signal is one-halfthat when the normal ({fraction (1/1)}) scan width is chosen. Inaddition, a period of the horizontal synchronization signal is alsoone-half the normal. As for the vertical synchronization signal, whileits amplitude is the same as that of the conventional scanning method,its period is one-half that of the normal scanning because the period ofthe horizontal synchronization signal is one-half that of the normalscanning. By setting the scan width smaller than that of the normalscanning as described above, the periods of the vertical and horizontalsynchronization signals can be made shorter, reducing the time requiredto retrieve one frame.

Further, combining the above-described scan line number reducing methodand scan width reducing method can enhance the effect of shortening thetime required to retrieve one frame. FIG. 8 schematically shows how thescan lines are scanned when the number of scan lines and the scan widthare both set smaller than the normal in the selection page of FIG. 3.Here, the normal (interlaced scanning) is chosen as the scanning order,⅛ (64 lines) as the number of scan lines, and ½ as the scan width.

FIG. 8A shows a range of image displayed on a monitor screen and anorder of scanning. The monitor screen is so set that the specimen imageis displayed on the entire screen when the number of scan lines isnormal (512 lines) and the scan width is {fraction (1/1)} (normal). Ifthe number of scan lines is set to ⅛ the normal and the scan width to ½,the vertical dimension of the image displayed is ⅛ the dimension of themonitor screen and the horizontal width is one-half the width of themonitor screen. Although the scanning order and the scanning range havebeen explained by referring to the monitor screen, the electron beam isalso swept over the specimen in a scan range that is narrowed in themain and sub-scan directions.

FIG. 8B and FIG. 8C schematically show a vertical synchronization signaland a horizontal synchronization signal, respectively, during thescanning operation. As for the horizontal synchronization signal, sincethe scan width is set to ½ the normal scan width, its amplitude isone-half that when the normal ({fraction (1/1)}) scan width is chosen.In addition, a period of the horizontal synchronization signal is alsoone-half the normal. As for the vertical synchronization signal, sincethe number of scan lines is set to ⅛, its amplitude is ⅛ of that whenthe normal number of scan lines (512 lines) is chosen. Further, becausethe amplitude of the vertical synchronization signal is ⅛ and the periodof the horizontal synchronization signal is one-half that of the normalscanning, the period of the vertical synchronization signal is {fraction(1/16)} that of the normal scanning. By setting the number of scan linesand the scan width smaller than those of the normal scanning asdescribed above, the periods of the vertical and horizontalsynchronization signals can be made shorter, reducing the time requiredto retrieve one frame.

An example application of this scanning is a dimension measurement. Thisis explained in an example case of measuring a bottom diameter of a holepattern shown in FIG. 9. It is assumed that the normal interlacedscanning is chosen as the order of scanning, the normal 512 lines as thenumber of scan lines, {fraction (1/1)} as the scan width, and 1 as thetotal frame number and that an image of a target to be measured isproduced in one scanning operation at a desired magnification anddisplayed on the monitor screen. An image of the normally scanned area,though its image resolution is low because the total frame number is 1,is displayed on the monitor screen as shown in FIG. 9A, so the user canrecognize the hole pattern being measured. An area 125 whose scan linenumber and scan width are reduced by the parameters chosen in theselection page of FIG. 3 is displayed on the screen over the rough imageto specify an area the user wishes to measure more precisely. Then, thescanning is performed for a desired number of frames specified in theselection page; image signals for the scanned frames are summed up; andan image with an improved S/N is displayed on the monitor screen. Then,vertical cursor lines 126 are aligned to a portion to be measured, asshown in FIG. 9B, for dimension measurement.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope comprising: an electron beam source;an electron beam scanning unit to focus an electron beam emitted fromthe electron beam source and sweep it over a specimen; a detector todetect secondary signals coming out from the specimen when it issubjected to the electron beam; and an imaging unit to form a specimenimage by summing up a plurality of image signals based on the secondarysignals detected by the detector; wherein a plurality of electron beamscanning patterns are selectably provided in which the electron beam isscanned over the specimen by the electron beam scanning unit; whereinthe plurality of scanning patterns have different time periods whichelapse after each electron beam radiation position in a scan area on thespecimen has received the electron beam until it receives the beamagain.
 2. A scanning electron microscope according to claim 1, whereinthe imaging unit forms a specimen image by summing up a plurality offrames of image signals and the scan area on the specimen is formed as aset of a plurality of parallel scan lines.
 3. A scanning electronmicroscope according to claim 2, further including a selection unitwhich is provided beforehand with the plurality of scanning patterns andselects one of the plurality of scanning patterns.
 4. A scanningelectron microscope according to claim 1, wherein the scanning patternsinclude a pattern in which a plurality of scans to be summed up areperformed on one scan line before the scanning operation moves to thenext scan line.
 5. A scanning electron microscope according to claim 4,further including a selection unit which is provided beforehand with aplurality of total frame numbers each as the number of image signalsthat are summed up to form the specimen image and which selects one ofthe plurality of the total frame numbers.
 6. A scanning electronmicroscope according to claim 1, wherein the imaging unit forms aspecimen image by summing up a plurality of frames of image signals anda plurality of scan line numbers are selectably provided each as thenumber of scan lines to be scanned by the electron beam scanning unit.7. A scanning electron microscope according to claim 6, furtherincluding a selection unit which is provided beforehand with theplurality of scan line numbers and selects one of the plurality of scanline numbers.
 8. A scanning electron microscope according to claim 6,wherein the number of frames can be selected.
 9. A scanning electronmicroscope according to claim 1, wherein the imaging unit forms aspecimen image by summing up a plurality of frames of image signals anda plurality of scan widths are selectably provided each as the width ofscan lines to be scanned by the electron beam scanning unit.
 10. Ascanning electron microscope according to claim 9, further including aselection unit which is provided beforehand with the plurality of scanwidths and selects one of the plurality of scan widths.
 11. A scanningelectron microscope according to claim 9, wherein the number of framescan be selected.